The present invention relates to a displacement machine such as a compressor, vacuum pump or the like operating according to the spiral principle and comprising two displacement elements that can be displaced relative to one another so as to execute a translational rotary movement.
Compressors, vacuum pumps and other displacement machines operating according to the spiral principle have been known for a fairly long time as can be seen for example in German Auslegeschrift No. 22 25 327 and German Offenlegungsschrift No. 26 03 462. The displacement is effected by two displacement elements or units, each of which essentially consists of a base plate with a spiral wall formed thereon defining a spiral recess. The spiral wall of each displacement element axially interpenetrates the spiral recess of the other element. By virtue of a generally circular but purely translational relative movement (parallel movement) of the two displacement units, the contact points move in the same direction or sense between the spiral walls and recesses, with the result that, depending on the rotational direction of the relative movement, the points move radially either from the outside to the inside or from the inside to the outside.
As is known, such displacement machines can be driven in two ways. The first way is to fix one displacement element and then cause the second element to execute the desired, generally rotary relative movement, via an eccentric drive means, generally a crank mechanism. In the second way, assuming a circular relative movement is desired, is to mount both displacement elements so that they can rotate the rotational axes being displaced by the desired degree of eccentricity. As soon as the spiral walls and recesses cover a circumferential angle of at least 2 .pi., permanent radial contact between the spiral walls exists in at least one point. As soon as the area of mutual contact between the spiral walls and recesses is at least more than double (circumferential angle 4 .pi.), at least two radial contact points permanently exist. In the latter case, sickle-shaped hollow spaces are formed between two contact points, in which a fluid can be conveyed in a unidirectional manner by means of the aforementioned relative movement.
The unidirectional conveying processes with low relative velocities of the displacement elements, wherein specific regions of the spiral walls and recesses are always associated only with the inlet region or outlet region, makes the use of pumps and compressors according to the spiral principle appear attractive in cases where high compression ratios should be produced without lubrication or with only a small degree of lubrication. Oilless displacement machines are preferred for reasons of servicing and operating costs, and environmental protection. There are also cases where oil is not only undesirable but is impermissible, e.g. on account of the danger of explosion.
It has in any case been found that the theoretically achievable high compression ratios and the simple manner of operation are difficult to realize in practice since a reliable and precise relative rotary motion and sealing at the radial contact points between the spiral walls and recesses is not easy to achieve. If a clean and smooth relative rotary motion at the contact points is not ensured, the result is increased wear and tear and localized heating at the spiral contours, and consequently the bearings weld together and seize up.
The main causes of an insufficiently clean and smooth relative rotary motion at the contact points may be attributed to:
(a) insufficient parallel guidance of the two displacement elements,
(b) insufficient machining accuracy of the spiral contours, and
(c) thermally produced contour variations or play on the spiral contours and at the contact points.
Known solutions to the aforementioned problems include, inter alia, highly accurate, adjustable crank drives as a parallel guidance system, precision machined spiral contours in air-conditioned areas, thermostatic regulation of the displacement units by means of a smooth circulation of cooling oil, and the like as can be seen for example in German Auslegeschrift No. 22 25 327. However, the aforementioned solutions involve a much higher expenditure in production costs than in the case of oillubricated pumps and compressors, such as rotary vane vacuum pumps and the like. For this reason spiral displacement machines have hitherto only been employed where, on account of the lack of alternative solutions, the high costs involved have to be borne.